1. Field of the Invention
The present invention relates to a frequency domain filter for modifying the transmission spectrum of an optical waveguide. The frequency domain filter of the invention is formed in a portion of optical waveguide which is modified to confer a filtering capability on it. The present invention applies to planar optical waveguides and to optical fibers. The principles of the invention are illustrated using the example of optical fibers, but the skilled person will know how to transpose this teaching to apply it as it stands to optical waveguides.
2. Description of the Prior Art
Filter optical waveguides include Bragg gratings in planar waveguides and in core sections of optical fibers. The Bragg gratings are formed by periodically modifying the refractive index of the material of the waveguide or the fiber by ultraviolet irradiation of the waveguide or the core of the fiber. The modification of the refractive index caused by exposure to light is referred to as the "refractive photo effect." This effect is permanent. The property of a material having an index that can be modified by irradiating it with light is referred to herein as its photosensitivity. The photosensitivity characteristics are related in the current state of the art to the presence of a germanium defect in the silica matrix of the waveguide or optical fiber. Other dopants can be used to render the waveguide or the core of the fiber photosensitive.
An advantage of germanium is that it is normally present in the core of optical fibers because it increases the refractive index of the core of the fiber relative to that of the optical cladding surrounding the core. This increase in the index, also referred to as the index step, guides the light signal in the core of the fiber. The same effect is used for waveguides.
Various layers of doped or undoped silica are successively deposited on the inside of a glass tube during the fabrication of an optical fiber preform and progressively adhere to the inside wall of the tube to constitute the various layers of the optical fiber. The diameter of a preform obtained in this way is greater than the diameter of the fiber, which is obtained by heating and drawing the preform.
A section of the waveguide or the core of the fiber which is to serve as a filter is then selectively and periodically exposed to ultraviolet radiation to form the Bragg grating. This irradiation brings about permanent local modifications of the refractive index. These modifications are related to a chemical and structural modification of the bonds of the germanium (or other dopant) atoms in the waveguide or the core. The variation in the value of the refractive index of the waveguide or the core of the fiber resulting from these modifications can be a few parts per thousand.
The grating then consists of a modulation of the refractive index along the section forming an attenuator filter.
Conventionally, when the refractive index modifications are perpendicular to the axis of the waveguide or the optical fiber, the quantity of light not transmitted by the filter is reflected into the waveguide or into the core of the optical fiber, with maximum reflection at the Bragg wavelength, which is determined by a resonance condition. In physical terms, the fundamental mode propagating codirectionally is coupled to the mode propagating contradirectionally.
Depending on the length of the section exposed, the period at which the modifications are reproduced along that section and the magnitude of the modification (the greater or lesser variation of the refractive index at the location of the modifications), the following transmission characteristics can be respectively modified: the width, the center frequency of the filter, and the degree of attenuation obtained (contrast).
If large variations of the index are photo-induced in the core of a fiber, the fundamental mode is also coupled to cladding modes, at shorter wavelengths. According to the paper D1="Optical fiber design for strong gratings photimprinting with radiation mode suppression" by E. DELEVAQUE et al, given to the OFC San Diego 1995, Post Deadline 5, conference, this can be avoided by doping part of the cladding close to the core with germanium. A fluorine codopant is added to the cladding to re-establish the index step.
In one particular application, attempts have been made to use such filters to compensate defects in the flatness of the gain of amplifiers used along very long haul optical links. Over very long distances, and in particular on submarine links, the attenuation per kilometer of the light waves in the optical fibers is such that optical amplifiers are required from place to place. Such amplifiers are known to have the unfortunate drawback of systematically favoring some of the frequency components in the band transmitted.
This phenomenon is accentuated by the fact that these optical amplifiers are used in wavelength division multiplexed (WDM) links in which various channels are transported by optical carriers at different frequencies in order to increase the global capacity and modularity of the system. Because of the phenomenon employed in the optical amplifier, the spectral response would be unacceptable if it were not regularly compensated. In the present application, what is most important is to flatten the gain of the erbium-doped fiber optical amplifiers. Other applications are naturally feasible.
This type of Bragg grating filter therefore has the disadvantage of acting as a partial reflector of the components of the amplified signal to which the filtering applies. Part of the optical signal at these frequencies is therefore reflected back into the optical amiplifier. As a result, the signal reflected by the filter causes interference in the amplifier section, but the signal back-scattered by the filter is also sent back in the line and degrades transmission characteristics.
The article D2 ="Wideband gain flattened erbium fiber amplifier using a photosensitive fiber blazed grating" by R. Kashyap, R. Wyatt and R. J. Campbell published in Electronics Letters, Jan. 21, 1993, vol. 29, No. 2, pages 154 through 156, in particular envisages the principle of inclining the fringes representative of the modulated index areas in order to prevent such reflection. This can be achieved by causing two beams from an argon laser doubled in frequency at 244 nm to interfere and by inclining the normal to the section serving as a filter relative to the exposure bisector of the two beams. A phase mask can also be used, principally generating two orders of diffraction (+1 and -1) and a very small zero order. The inclination is 8.degree. in the above article, for example. The advantage of the inclination is that it eliminates reflection. This is because the effect of the inclination is to couple the fundamental mode propagating codirectionally with contradirectional cladding modes. These cladding modes are very quickly absorbed by the cladding. The spectral envelope of the set of frequency components in these various cladding modes can then be used as characteristic of a fiber used to compensate the gain of the optical amplifiers.
The disadvantage of this technique lies in the selectivity of the filter. This is because, using standard telecommunication fibers, it is not possible to obtain a filter band less than 20 nm, for example, using a Bragg grating filter of this kind with inclination of the index modifications. It is possible in theory to alter the diameter of the core to reduce the bandwidth of the filter. The filter is therefore more selective if the core diameter is greater, for example 9 .mu.m instead of 3 .mu.m. However, this increase in diameter is limited. Furthermore it has a number of drawbacks, including that of requiring adaptation sections between a fiber with a large diameter core and a fiber with a standard diameter core (in the order of 9 .mu.m). These adaptations are difficult to make.
The attenuation by the cladding modes is improved, which is in accordance with the stated objective, but the length of the grating can no longer reduce the bandwidth of the filter. In practice reducing the angle makes the filter more selective but at the same time increases residual emission by reflection, of the upright fringe type. In contrast, increasing the inclination of the angle reduces the effect of the reflection phenomenon but increases the bandwidth of the filter, i.e. reduces the selectivity of the filter. The compromise arrived at is not satisfactory in all cases and there is room for improvement.
A second problem with this type of filter is connected with filter rebound in a low-frequency band, i.e. a band at a greater wavelength, close to the wanted band in which the filtering is applied. This rebound is due to the residual reflection referred to above in the fundamental mode. Initially this rebound is not a problem because prior art optical amplifiers have a limited bandwidth, outside which the filter rebound occurs. It must nevertheless remain low. However, in other applications, and in particular in terrestrial applications, the filter is used selectively to attenuate different components in the wanted band. The rebound will therefore also be in the wanted band. Filter rebound is therefore a problem in these other applications.
Thirdly, it has already been pointed out that the attenuation is in fact merely the envelope of the attenuations of different spectral components. This means that, within the envelope, some spectral components are effectively filtered while others are filtered to a lesser degree, or even not at all. This is due to the discrete nature of the cladding modes. Under these conditions, the filter envelope corresponds to a set of relatively narrow bandwidth discrete filters separated from each other by frequency gaps in which there is no filtering. Thus a filter of this kind cannot be used to equalize correctly the gain of the optical amplifiers.
The drawback of the compromise referred to above in connection with the teaching of document D2 is addressed in document D3="Ultra Narrow Band Optical Fiber Sidetap Filters" by M. J. Holmes, R. Kashyap, R. Wyatt and R. P. Smith, Proc. ECOC 1998, pp. 137-138, Madrid. Sep. 20-24, 1998. Document D3 teaches increasing the selectivity of a Bragg grating filter by forming a slightly inclined grating in the cladding of the fiber (and not in the core). Because of the absence of fringes in the core, retroreflection losses in the fundamental mode are considerably reduced, even at very small angles of inclination (3.degree. rather than the 8.degree. of document D2).
In French patent application 98 06904 (A1), the first problem is solved by causing the cladding to participate in the phenomenon of interference employed in the filtering process. According to the teaching of application A1, the Bragg grating is formed in the core and in the cladding. It is therefore formed over a greater diameter than the core of the fiber and the filter is therefore rendered more selective. The spectrum of the filter is then easily controlled by altering the diameter of the part of the cladding that contributes to the radiative coupling.
The present invention addresses the third problem referred to above with reference to document D2, namely the modulation of the response of the filter as a consequence of the discrete nature of the cladding modes. To increase the filter power (contrast) of a Bragg filter, either the amplitude of the photo-induced refractive index variations can be increased or the length of the filter can be increased. The present invention addresses the problem of increasing the length of the filter, which has the unwanted effect of narrower coupling to each of the cladding modes and therefore greater modulation of the filter response.